The goals of our work are to determine the extent to which mRNA populations of adult, immature, regenerating and neoplastic rat liver may differ quantitatively and qualitatively from each other and to identify messenger RNAs (mRNA) and sets of active genes which might be specific for a certain growth pattern. We have established that a) the proportion of the non-repetitive genome transcribed is the same in normal and regenerating livers and b) that polyadenylated polysomal mRNA of regenerating and pre-neoplastic rat livers differ from the corresponding normal liver RNA only in the frequency at which some RNA sequences are represented. Qualitative differences, if they exist, are likely to be restricted to the expression of a very limited set of genes.